Trunnion Alignment

Just because you align the blade at 90 degrees doesn’t mean it will stay that way when you tilt it to 45 degrees. On most table saws, the entire blade carriage rides in semi-circular tracks called “trunnions”. The blade is tilted as the assembly travels along these tracks. The “axis of rotation” for the blade tilt mechanism is supposed to run along the surface of the table where the blade comes through the table insert (throat plate). This way, the table insert won’t interfere with the blade even when it’s tilted. However, inaccuracy in machining, or stress relief in the castings often causes this axis of rotation to shift so that it no longer runs along the surface of the table. So, as the blade is tilted, it gradually gets out of alignment with the miter slot. Fortunately, this process should only be required once for the life of your saw.

Why

The following symptoms are common when making bevel cuts on a saw when the trunnions are not properly aligned:

Burning of the cut edge

Board wandering away from the fence during a rip cut

Sawdust being thrown up at the operator

Kickback

These are the exact same symptoms that occur for normal (90 degree) blade alignment. If you see any of these symptoms when you tilt the blade (making bevel cuts), then you need this procedure.

How

First, make sure that the standard blade alignment (blade at 90 degrees) is accurate. Then, tilt the blade to 45 degrees and check the alignment.

The measurements

The best way to do this is to tilt the dial indicator as shown in the photo above. The most accurate readings always result when the plunger of the dial indicator is at right angles to the surface being measured. Use the same method that was described for standard blade alignment - mark a dot on the blade and take all measurements with the stylus of the dial indicator on that spot - rotating the blade as necessary. The error measured in this step will represent a combination of horizontal (90 degree) and vertical (tilt axis) misalignment. If you don’t perform the standard 90 degree blade alignment accurately, then it will influence your readings when the blade is tilted and invalidate the procedure.

Measurements can be made with the dial indicator horizontal as shown above but I don’t recommend it. Many sources of error can creep into your readings making them inconsistent and very frustrating. The dial indicator will have a tendency to lift during the measurement process so be sure to check that the jig remains flat on the table. Since the dial indicator is not perpendicular to the surface being measured, the readings will be exaggerated by 1/cos(45). This turns out to be equivalent to the vertical (tilt axis) misalignment so no further correction is needed.

Make a note of the change in reading between your two measurement points as well as the direction of the change. In this case it’s about 0.006 inches and it’s higher at the trailing edge of the blade. To determine the vertical (tilt axis) component of the misalignment, multiply the measured error by 1.414 (the square root of 2). The result, 0.0085″, is the amount of tilt axis error influencing the misalignment of your blade. When measured with the dial indicator horizontal, the reading is a little more than 0.008 inches. Correcting the error will involve inserting some shims and you will need this information to calculate the proper thickness of the shims and where to install them.

The second number you will need is the distance between the two measurement points. In this case, it’s about 8 inches. When measuring with the dial indicator horizontal, the distance between the measurement points is also about 8 inches.

The third number you will need is the distance from front to back between table bolts (cabinet saw) or trunnion bolts (contractor’s saw). For my saw it’s about 20 inches.

The Calculation

To figure out how thick the shims need to be, first calculate the amount of change in reading per inch of measurement. The vertical (tilt axis error) we calculated above is 0.0085″. This was calculated from a measurement over an 8 inch span. That’s 0.0085/8 or about 0.001 inches change per inch of measurement.

The thickness of the shims will be the distance between front and rear table bolts (cabinet saw) or trunnion bolts (contractor’s saw) multiplied by the “change per inch” calculation. So, 20 x 0.001 = 0.020 inches.

Installing the Shims

Make your shims out of metal. If you make them out of wood, plastic, or cardboard, then they will compress over time and you will lose your alignment. If a large amount of change is required, you will want to check the thickness of washers. Soft drink cans make good material to cut thinner shims from. Aluminum foil can be used for making very fine adjustments to the overall thickness.

Cut your shims in the shape of a “U” so that they can fit around the mounting bolt. Dial calipers will help you to measure the thickness of the shims so that you can start out with something close to what you need. But, as you install them and tighten the bolts some compression may occur.

If you have a cabinet saw, then the shims have the effect of raising the table. They go between the table and the top of the cabinet underneath the bolts that hold the two together. Place them under the two bolts at the front of the saw if the reading at the leading edge of the blade was higher than at the trailing edge. If the reading was higher at the trailing edge of the blade, then the shims should be placed underneath the two bolts at the back of the saw.

I’ve found it handy to put a dial indicator on the table surface above one of the mounting bolts. This will allow you to monitor the amount that the table is being raised.

If you have a contractor’s saw, then the shims have the effect of lowering the blade carriage. They go between the trunnions and the table. If the reading at the leading edge of the blade is higher than at the trailing edge, then you need to shim the front trunnion. Working on the rear trunnion is much easier than the front trunnion. So, to save your knuckles from extreme torture, turn the saw upside down and deliberately over-shim the front trunion. Then, turn it right side up and figure out how much shimming needs to be done to the rear trunnion to correct the misalignment.

If the reading was higher at the trailing edge of the saw, then place the shim under the trunnion at the back of the saw. It’s best to locate the shims directly underneath the bolts.

Checking the results

All of this shimming is likely to mess up the standard (blade at 90 degrees) alignment. So, start by checking it and make any adjustments that are necessary. Then, tilt the blade to 45 degrees and check for any alignment error. If you did a good job making measurements and installing shims, the error should be significantly reduced or even eliminated.

Re: Trunnion Alignment

Its a fancy jig when compared to my sticks but works the same. I have read several things on saw tune up and they all was pretty much the same, none of them talked about the above step. Mine was .008 out of adjustment and I got it down to .002. I need to go back and get it closer, no reason for it to be that high of number.

Re: Trunnion Alignment

Probably the reason it's not mentioned much in the first place is that it's not perceived as a serious issue.

The method to check for this condition is static, and may change during dynamic loading as well as being next to meaningless if you've got a lot of axial runout like I do in my inexpensive benchtop saw. I think most guys are satisfied by checking the angle of cut, the squareness to an edge, and an even distribution of kerf marks on the cut face, why take it any further?

Don't ask advice on federal firearms law from people on the internet unless you like jail food.